I will give my input. I had a similar situation to you. I took Biochem I (notoriously hard at my school/ the weed out class), Cell Bio (again a weed people out kind of class), and comparative anatomy and physiology (average difficulty at my school).. all in one semester.
I will say that for the two semester before that I had all A's or A-s... the three semesters after, same thing. But THAT semester I ended up with a B+ in all three of those classes, and a B in my non-science class that I was taking.
Looking back, its not like it scarred me, and I dont have horrible nightmares about that semester heh. However, I do remember it being very time consuming. I remember that was a particular semester where I had relationship strain with my significant other because of my commitment to school. I remember pretty much studying a lot...
The reality that I found was that I just could not give each of those notoriously challenging courses the respect that they deserved. But in the end, is a couple of B+'s the end of the world? I doubt it... it worked out ok in my case.
One thing I would support for taking all of these courses is that it actually was very nice to be taking a ton of science classes at the same because of the overlap. I found that while my grades were slightly lower than they would have been if I was taking cushion classes, I found that this was the semester that REALLY made me passionate towards science. A) because thats pretty much all I was doing, it was hard not to start liking some of the stuff and B) (probably the better reason) there was an immense amount of overlap between the courses that overall strengthened my learning. So while I did not have the time necessarily make all of my work PERFECT and get straight A's, I would say the meat of all of the science that I know and still use today was founded on the semester that took these three challenging courses. Things I learned in biochem would spill over to anatomy, and things in anatomy would spill over to cell bio etc. I think while challenging, it is really conducive to learning the material well.
Would I do it again? most likely... I would say if it will make your senior year easier and stuff, then go for it (this was my mindset but I ended up taking on extra hard courses during my senior year anyways haha). But just be aware that you will essentially be committing to 6 classes. A ton of time for each of the 3 lectures, and a ton of time for each of the 3 labs.
